The small subunit of the bacterial ribosome is a complex organelle composed of one molecule of RNA and 20-21 different proteins which is responsible for recognition of the points on messenger RNA at which protein synthesis is initiated and the specific binding of the correct charged tRNA molecule corresponding to each codon of the messenger RNA during protein synthesis. In order to carry out these functions the small subunit interacts specifically with at least three initiation factors, charged tRNA molecules, mRNA, and the large subunit of the ribosome. In order to understand the molecular mechanisms by which the small subunit accomplishes its role in protein synthesis it will be necessary to determine the three dimensional structure of the ribosome. Electron micrographs of the small ribosome subunit show two distinct views of the ribosome which may be combined to give a three dimensional model of the overall shape of the subunit. When antibody molecules which specifically recognize a single ribosomal protein are bound to the subunit and visualized by electron microscopy, the location of the protein in the subunit may be deduced. It is proposed to determine the location of each of the ribosomal proteins in the subunit by this method and further to locate specific regions of some of the proteins through the use of antibodies specific for particular portions of the primary sequence of the protein.